Review of pumping by thermal molecular pressure

Pumping energy is supplied by temperature changes alone. A general feature of such pumps is that the upper pressure limit is reached when the mean fre e path becomes small relative to the physical dimensions of the pump in the region of the temperature transition. Thus, the upper pressure limits of t hese pumps have been determined by the microfabrication limits of the day; they have operated at relatively low pressures, with low throughputs, and h ave not become main line pumps. In recent years, however, Micro-Electronic- Mechanical Systems (MEMS) has introduced a whole new level of miniaturizati on to devices in general, including vacuum devices, and hence has raised th e upper pressure limits, and thus the throughputs of thermal molecular pump s to atmospheric levels. The purpose of this article is to isolate the vari ous physical manifestations of thermal molecular Dumps, which have been rea lized over the years. The general pumping phenomenon has had various names: Knudsen compressor; thermal transpiration; thermal creep; thermodynamic, t hermolecular, thermal molecular, and accommodation pumping. This multiplici ty of terminology can cause some confusion and it is one of the aims of the article to simplify the situation. We have chosen the title "Pumping by Th ermal Molecular Pressure" following the terminology of Knudsen. Broadly spe aking, it is found that these pumps divide into two classes: (a) those requ iring no specially prepared surfaces, (b) those in which special surfaces a re essential. The latter have no low pressure limit. A table is assembled c omparing pumps which have been built and tested, rather than those calculat ed on paper. Scaling rules for multiple stage pumps in series, based on res ults obtained for single-stage pumps are presented. A Knudsen compressor in series with an accommodation pump already promises operation from atmosphe re to indefinitely low pressures, whereas an accommodation pump alone may b e able to cover this range in the future. A number of potential application s of the technology such as small gas chromatographs and small valves are n oted. Despite this complexity, thermal molecular pressure devices all have the compelling advantage that there are no moving parts nor any fluids in t he vacuum. (C) 2000 American Vacuum Society. [S0734-2101(00)01104-0].